1. Field of the Invention
The present invention generally relates to optical illumination systems, and more particularly to a polarized illumination system having a novel spatial integrator, including variations for controlling aspect ratio, the system being suited for use with electronic projection displays, particularly of the liquid crystal display type.
2. Description of the Prior Art
With the increasing use of liquid crystal display (LCD) devices in projection systems, there is a requirement for an efficient source of polarized light. Polarized light has been traditionally produced by an absorbing polarizer or a polarizing beam splitter (PBS) cube. For polarizing beam splitters, half of the light is reflected and this reflected polarized light is either thrown away, or converted to the same polarization as the transmitted beam. When converted, the reflected beam is then redirected to the LCD, along with the transmitted beam, to achieve a polarized light source that is brighter than one using a simple absorbing polarizer.
An early application of converting polarized light in illumination systems was for automotive headlamps, as described by Zehender in "Headlights for Motor-Vehicles with Polarized Light," Lichttechnik, pp. 100-103 (1973). In this early design, no attempt was made to achieve spatial recombination of the separate beams, or to control the beam cross-section. With the advent of LCD projection systems, this technology achieved renewed interest, for example as described by Imai et al., "A Novel Polarization Converter for High-Brightness Liquid Crystal Light Valve Projector," SPIE Proceedings, vol. 1225, pp. 52-58 (1990), and Shinsuke et al., "A Polarization-Transforming Optics for a High Luminance LCD Projector," Proceedings of the SID, vol. 32/4, pp. 301-304 (1991).
Japanese Patent Application (Kokai) No. 61-122626 describes a polarizing illumination device, that spatially integrates the separated beams at the LCD plane by means of a pair of wedge prisms. As shown in FIG. 1, the device uses a light source 1, collimator 2, PBS cube 3, right angle prism 4, half-wave retarder 5, wedge prisms 6 and 7, LCD panel 8, and projection lens 9. There must be a considerable distance between the wedge prisms and the LCD, since the beams are converging, and the incidence angle on the LCD must be kept small, which unduly restricts use of the device.
U.S. Pat. Nos. 4,913,529 and 4,969,730 describe converting polarized light projection illumination systems using polarizing plates or a PBS cube, combining the separated beams at the LCD with steering prisms. U.S. Pat. No. 5,381,278 uses converging and diverging lenses to redirect the separated beams to the LCD panel. Japanese Patent Application (Kokai) No. 71-99185 uses dual polarizing beam splitters, achieving a beam of oblong cross-section, but with no spatial recombination of the separate beams. European Patent Application No. 615,148 achieves polarization conversion and spatial recombination by recycling light backwards to the light source reflector, as does European Patent Application No. 573,905, assigned to Minnesota Mining and Manufacturing Co. (3M--assignee of the present invention). U.S. Pat. No. 5,181,054 achieves spatial recombination of the separated beams by sending the beams through the LCD in opposite directions, as does U.S. Pat. No. 5,428,469 (also assigned to 3M). U.S. Pat. No. 5,446,510 achieves a common collimation angle for the separate beams, but without any spatial integration.
U.S. Pat. Nos. 5,042,921 and 5,124,841 describe polarization converters with spatial integration achieved by refracting microprisms. The polarization converter described in European Patent Application No. 463,500 preserves the aspect ratio of the original beam, but requires the use of two LCD's, and European Patent Application No. 456,427 matches the beam size to the LCD panel by the use of back reflections from a lamp reflector having a rectangular exit aperture. The efficiency of these systems is limited by the complexity of the optics, quality of the reflectance coatings, degree of spatial recombination and beam shaping, and high chromatic dispersion of the refracting elements.
Spatial integration of the separate beams is important since the separated beams usually differ in intensity and color temperature. Collimation of the beam incident on the LCD is important, since most displays of this type work best with a common and low incidence angle of the illuminating light. Also, collimated light can be more efficiently focused by a field lens to the projection lens. Beam shaping is important to transmit maximum light through the rectangular aperture of the LCD. Lastly, compactness is desirable to reduce the size of the projector unit. None of the foregoing systems provides optimum performance in all of these areas.